1. Field of the Invention
The present invention relates to an ultrasonic printer in which convergent ultrasounds are radiated to emit an ink near a convergent point of the convergent ultrasounds in the form of an ink droplet and deposit the ink droplet on a recording medium such as a paper sheet, thereby performing a recording on the recording medium with multiple ink dots.
2. Description of the Related Art
Recently, there has been widely applied an ink jet printer adapted for recording by means of directly emitting a particle of ink or an ink droplet onto a recording medium such as a paper sheet. Such an ink jet printer has many advantageous points such that a high speed printing is available, a lower noise printing is available, there is little a restriction to the recording medium, it is easy to provide a coloring, and so on.
In this type of ink jet printer, on the other hand, it happens during a recess in printing that a viscosity of ink on a nozzle of an ink jet recording head is increased, or during the printing that a bubble enters the nozzle. Such increase in viscosity of the ink and the occurrence of the bubble in the nozzle can cause such troubles when the printing starts, as hard ejection of the ink, printing with some dots missing and clogging on the nozzle when the ink hardens, and such troubles would eventually cause a deficient recording head. In view of the foregoing, there are taken the necessary measures or backup such as a capping in which nozzles are capped, when the printing is not performed, to prevent evaporation of water of the ink, a wiping in which excess ink on the nozzle is wiped, and a suction purging in which the nozzle is covered with a suction cap, when a power supply is turned on or when needed, to remove inks of which viscosities have been increased or inks into which bubbles are mixed. To enable the backup operation, however, the prior printers involve such problems that the structure of the printers is complicated and in addition, cost of the product increases.
Further, the prior printers suffer from the following drawbacks. Part of inks adheres an edge of an orifice of a nozzle with stain and hardens thereon, so that a flying direction of inks is varied to cause a dot deviation. This leads to a disturbance of print, and in case of a color printing leads to the change of hue.
Furthermore, the prior ink jet printers each are provided with an ink chamber and nozzles, which adopt a scheme in which the ink chamber is compressed with a piezoelectricity to eject inks from the nozzles, or another scheme in which the ink chamber is heated by a heater to emit inks. However, according to such prior ink jet printers, it takes a lot of time to refill inks of a nozzle chamber on a repetitive basis, and thus there is a restriction in time to eject the successive ink droplet.
In case of a nozzle fashion, since a diameter of the nozzle is fixed, a size of the ink droplet is substantially determined. Thus, in this case, it is difficult to change a size of print dots.
Further, in case of a nozzle fashion, if clogging occurs on even one of the nozzles, the recording head becomes unavailable in its entirety. Consequently, in case of the nozzle fashion, there is often adopted a throw-away type of head in which the head and an ink tank are formed in a single unitary body. Thus, in this case, since there is a structure as articles for consumption, the print cost or running cost increases.
To solve these problems, it is desired to provide a new type of printing system which needs no nozzles, and be simple in structure and inexpensive.
As an example of a printing system satisfying these requirements, recently, there is proposed an ultrasonic printer. An acoustic lens or the like are used to project an ultrasonic acoustic beam toward a free surface of a pool of liquid from beneath so as to focus on the surface of the pool, so that individual droplets of liquid are released from the surface of the pool. This principle has been applied to the ultrasonic printer, using ultrasonic acoustic beams to release small droplets of inks from pools of ink and to eject the droplets onto a recording medium such as paper sheets for printing.
FIG. 59 is a perspective view of a recording head of the prior art ultrasonic printer, and FIG. 60 is a cross sectional view of the recording head shown in FIG. 59, with the recording head being submerged in a pool of ink for operation (refer to U.S. Pat. No. 4,751,530).
Referring to FIGS. 59 and 60, the recording head comprises an array of precisely positioned spherical acoustic lenses 12 for launching a plurality of converging acoustic beams into a pool of ink. The acoustic lenses 12 are defined by small, generally spherically shaped indentations which are formed in the upper surface of an acoustic solid substrate 10. An ultrasonic acoustic transducer 14 is deposited on or otherwise maintained in intimate mechanical contact with the opposite or lower surface of the substrate 10 in such a manner that it is located over against the associated acoustic lens 12. When the ultrasonic acoustic transducer 14 is excited to generate ultrasonic acoustic waves, as shown in FIG. 60, the ultrasonic acoustic waves are propagated through the substrate 10 and curved by the acoustic lens 12 in a direction to converge, since the substrate 10 is constructed of a material having a higher velocity of sound relative to the ink 16, so that the ultrasonic acoustic waves are converged near the free surface 16a of the ink 16. In this manner, an ink droplet is ejected from the free surface 16a toward a recording sheet. The ejected ink droplet has a dot diameter which is approximately the same as the spot diameter of the converged ultrasonic acoustic waves. With such an ink droplet, the corresponding one dot of recording is implemented. When the ink droplet is deposited on the recording sheet, the size of the formed ink droplet will be expanded approximately twice as large as the size of the particle of the ink droplet.
FIG. 61 is a schematic diagram showing a functional structure of another embodiment of the prior art ultrasonic printer used to print bar codes (refer to U.S. Pat. No. 4,308,547).
Referring to FIG. 61, Ink 22 held in a reservoir 20 is applied to an ink conveying belt 26 by a roller 24. The ink conveying belt 26 is formed in an endless structure and circulated by rollers 28. An array of ultrasonic acoustic transducers 30 is centered on the ink conveying belt 26. The ultrasonic acoustic transducer 30 in the shape of a cylindrical segment is mechanically coupled to a wedge shaped acoustic medium as a concentrator. When ultrasonic acoustic waves are radiated from any of the ultrasonic acoustic transducers 30, the ultrasonic acoustic waves are concentrated owing to the cylindrical configuration of the transducer, so that an ink droplet is ejected from the ink conveying belt 26 via a slit 34 to a recording sheet 36, thereby implementing the recording on the recording sheet 36.
In the ultrasonic printers shown in FIGS. 59 and 60, the recording head comprises an array of precisely positioned spherical acoustic lenses 12 and an array of ultrasonic acoustic transducers 14, each element being associated with one dot. There are needs to supply to the respective transducers energy sufficient for ejecting ink droplets, and to focus the ultrasonic acoustic waves to a sufficiently small spot, for example, about 0.03 mm .phi., in order to attain a higher resolution. Consequently, it is necessary for configurations of the ultrasonic acoustic transducers 14, and the acoustic lenses 12 to have sizes such extent that the conditions as noted above are satisfied, for example, 1 mm angle and 1 mm .phi., respectively. By the way, there is a conflict between arranging 1 mm angle of ultrasonic acoustic transducer 14 and 1 mm .phi. of acoustic lense 12 per dot and implementing a high resolution printer capable of performing recording of, for example, 0.06 mm in dot pitch. In order to solve this conflict, there has been proposed such a system that multi recording heads (e.g. 16 rows) as shown in FIG. 59 are arranged in a stagger-like configuration so that the dot pitch is less than an arrangement pitch of the ultrasonic acoustic transducer. However, the provision of such many recording heads will involve an enlargement of the printer and a dramatical cost-up in manufacture.
Also in the prior ultrasonic printers shown in FIG. 61, it is necessary for each of the ultrasonic acoustic transducers 30 to radiate ultrasonic acoustic waves having energy which is sufficient to eject ink droplets. This results in a significant large arrangement pitch. Further, if the length (the size in a horizontal direction in FIG. 61) of the ultrasonic acoustic transducer 30 is elongated and the arrangement pitch is shortened by the corresponding elongated length, the spot diameter will be expanded as the arrangement pitch is shortened, since the spot diameter of the ultrasonic acoustic waves in an arrangement direction depends on a directivity of the ultrasonic acoustic waves. Thus, while the ultrasonic printer shown in FIG. 61 is suitable for a rough printing such as bar codes, it is difficult to apply it to the ultrasonic printer capable of implementing a higher resolution as mentioned above.